1. Field of the Invention
The present invention relates to bimetals frequently used in thermal control devices and the like, and more particularly to a bimetal which has a high bending characteristic by using Ni--Co--Fe type low thermal expansion alloys consisting of a specific composition range as a low thermal expansion alloy, and is capable of enlarging a proportional temperature range.
2. Description of the Prior Art
In general, a bimetal which is formed by bonding and laminating at least two or more kinds of metal or alloy having different thermal expansion coefficients by a suitable method and rolling into a plate or wire, has a bending characteristic due to temperature changes responsive to the difference in thermal expansion coefficients, and is used in many applications as in thermal control devices and for accomplishing color adjustment of a color television and the like.
As the metal or alloy constituting the bimetal, a Ni--Fe alloy is used as a material having a low thermal expansion coefficient, and as a material having a high thermal expansion coefficient, Ni, a Zn--Cu alloy, Ni--Cr--Fe alloy, Ni--Mn--Fe alloy, Ni--Mo--Fe alloy, Cu--Ni--Mn alloy and the like are used in response to the working temperature ranges.
Also, for adjusting an electrical resistivity of the bimetal responsive to its use, it is known to bond an intermediate metal or alloy layer such as Cu, a Cu alloy, Ni, or the like is between the low thermal expansion alloy an d the high thermal expansion metal or alloy.
Furthermore, when necessary it is known to include a corrosion-resistance coating such as a stainless alloy and the like or a getter material coating such as Al, Zn and the like on the bimetal surface.
As described above, though the conventional bimetal is constituted by bonding the low thermal expansion alloy and the high thermal expansion metal or alloy with or without interposing an intermediate metal or alloy layer therebetween, in any cases an Ni--Fe alloy is used as the low thermal expansion alloy.
A 36 wt % Ni--Fe alloy is known as the most common alloy used in the bimetal as the Ni--Fe alloy (JIS c 2530). However, the 36 wt % Ni--Fe alloy reaches at the transition point at about 200.degree. C. and the thermal expansion becomes considerably large at temperatures above the transition point, it is disadvantageous in that a proportional temperature range as the bimetal is restricted within the range of about 150.degree. C. or less.
A 42 wt % Ni--Fe alloy is known as the Ni--Fe alloy having the higher transition point of thermal expansion than the 36 wt % Ni--Fe alloy.
The thermal expansion transition point of the 42 wt % Ni--Fe alloy is about 350.degree. C., and the proportional temperature range as the bimetal can also be enlarged to about 350.degree. C.
However, since the thermal expansion of the 42 wt % Ni--Fe alloy is substantially fairly large as compared with that of the 36 wt % Ni--Fe alloy (for example, when compared by a mean thermal expansion coefficient in the temperature range of 30 to 100.degree. C., the 42 wt % Ni--Fe alloy about 4.times.10.sup.-6. K.sup.-1 and the 36 wt % Ni--Fe alloy is about 1.7.times.10.sup.-6. K.sup.-1), it is disadvantageous in that a bending coefficient as the bimetal is small.
As previously described, the bimetal is used very widely, and in recent years the working temperature range has increased for example, in thermal control devices, and a high-precision control in the high temperature region is required, thus a linear bending characteristic in the high temperature region is also required for the bimetal. That is, it is indispensable to enlarge the proportional temperature range.
As thermal control devices and the like are required not only to be smaller and lighter, also to be more durable, it is also desirable for the bimetal to improve its bending coefficient further.
However, in the bimetal proposed at present using the 36 wt % Ni--Fe alloy or 42 wt % Ni--Fe alloy as the low thermal expansion alloy, since it has such disadvantages as described before, the recent demand can not be satisfied.
It is therefore an object of the present invention to provide a bimetal which solves the aforementioned disadvantages and has a similar or larger proportional temperature range than the bimetal using the 42 wt % Ni--Fe alloy as the low thermal expansion alloy, and a higher bending characteristic which is a larger bending coefficient than the bimetal using the 36 wt % Ni--Fe alloy.